Government spokesman Sutopo Purwo Nugroho told a news conference in Jakarta that the danger zone affects 22 villages and about 90,000 to 100,000 people.

He said about 40,000 people have akready evacuated but others have not left because they feel safe or don’t want to abandon their livestock.

“Authorities will comb the area to persuade them,” he said. “If needed we will forcibly evacuate them.” About 25,000 people were already living in evacuation centres after an increase in tremors from the mountain in September sparked an evacuation.

Today, the alert level was raised to “stage four” as experts reported a large eruption was now “imminent.”

Geological agency head Kasbani, who goes by one name, said the alert level was raised at 6am (local time) because the volcano has shifted from steam-based eruptions to more powerful magmatic eruptions.

“Continuing plumes of smoke are occasionally accompanied by explosive eruptions and the sound of weak blasts that can be heard up to 12km (seven miles) from the peak,” said the Disaster Mitigation Agency (BNPB) in a statement.

“Rays of fire are increasingly visible from night to the following day. This indicates the potential for a larger eruption is imminent,” it said.

Ash-covered roads, cars and buildings near the volcano in the northeast of the island, while a red glow of what appeared to be magma could be seen in photographs released by Antara, the state news agency.

Gede Suantika, an official at the volcanology and geological disaster mitigation agency, said: “The activity of Mount Agung has entered the magmatic eruption phase.

“It is still spewing ash at the moment but we need to monitor and be cautious over the possibility of a strong, explosive eruption.”

Bali, famous for its surf, beaches and temples, attracted nearly five million visitors last year but business has slumped in areas around the volcano since September when Agung’s volcanic tremors began to increase.

Agung is one of over 120 active volcanoes extending the length of Indonesia.

When it last erupted in 1963 it killed more than 1,000 people and razed several villages.

Australia’s Bureau of Meteorology’s Volcanic Ash Advisory Centre (VACC) in Darwin issued maps showing an ash cloud heading southeast over the neighbouring island of Lombok, away from Bali’s capital, Denpasar, where the main international airport is located.

Indonesia also upgraded its Volcano Observatory Notice for Aviation (VONA) to red, its highest warning, and said the ash-cloud top could reach 19,654 feet (6,142 metres) or higher.

“The volcanic ash has only been detected in a certain area,” the airport and other officials said in a joint statement.

Yunus Suprayogi, general manager of Bali airport operator Angkasa Pura I, said food and entertainment would be provided as well as extra bus services if conditions changed and passenger numbers increased.

The airport would also “make it easier” for passengers to seek refunds and make other arrangements, he said, while noting that airlines had their own rules.

Here's what travellers need to know

What happened?

Ash from the volcano reached the Bali airport’s airspace, triggering its closure early Monday. More than 400 flights to and from Bali’s airport were cancelled and nearly 60,000 travelers have been stranded. Travel disruptions will ripple around the globe as the island is one of Asia’s top destinations, attracting 5 million visitors a year. December through the first week of January is one of the island’s busy periods. The airport’s initial closure is for 24 hours until early Tuesday but there could be prolonged or repeated disruptions because the volcano may spew ash for weeks or have even bigger eruptions.

Trapped in paradise?

Travelers can still leave Bali, though probably at significant cost, by taking a ferry from Gilimanuk port to Banyuwangi on Java island to the west and then traveling by car, train or plane to the Indonesian capital Jakarta for international connections. Another possible route is a ferry to the neighboring island of Lombok and then a flight to Java though ash temporarily closed Lombok’s airport on Sunday and there could be further closures. The government says it has provided 100 buses to transport people from the international airport to ferry ports. Alternatively, visitors could wait to see if the airport reopens though there’s likely to be long waiting lists for flights when it does. Indonesia’s tourism ministry said member hotels of the Indonesia Hotel and Restaurant Association will provide a night’s free accommodation to guests affected by the airport closure.

Heading to Bali?

Some governments are advising citizens to defer travel to the island. Malaysia’s foreign ministry said all but non-urgent travel should be delayed while Australia, a big source of tourists for Bali, said people should be aware that ash clouds could also close other airports in Indonesia depending on weather conditions. A spokesman for major Japanese travel bureau JTB Corp. said all tours set to depart Monday from Japan were canceled because of the airport closure. Yoshimi Tajima said some people can’t change their travel dates and are switching to other destinations while others are changing the dates. “What will happen from tomorrow is still unclear,” she said.

Local conditions

The popular tourist areas of Kuta and Seminyak and the airport are about 70 kilometers (43 miles) from the volcano and largely safe even with a major eruption though masks and goggles could prove necessary if there’s significant ash fall. Authorities have ordered 100,000 people to leave a danger zone that extends 10 kilometers (6 miles) from the crater in places and tourists shouldn’t enter it. The area, which includes a much-visited Hindu temple on the slopes of the volcano, will be prone to deadly mudflows and lava. The risk of mudflows, which combine water and volcanic debris, is high because it’s the wet season in Bali. Authorities have warned people to stay away from rivers.

After resuming flights on Sunday morning, Virgin Australia again cancelled flights on Sunday afternoon following a change in the aviation colour code from orange to red.

“Due to the significant volcanic ash and current weather conditions, we have made the decision to cancel the rest of todays flights to and from Bali as a precautionary measure,” Virgin said in a statement on its website.

AirAsia also cancelled its remaining flights to Bali and Lombok.

Qantas and Jetstar flights were continuing as of Sunday afternoon but Jetstar warned on its website that flights could be subject to change at short notice for safety reasons.